Climate-driven population dynamics, growth, and phenology of the moon jellyfish Aurelia coerulea in the Mediterranean thau lagoon

Global warming is reshaping marine ecosystems by altering species distributions, trophic interactions, and the timing of life-cycle events. Gelatinous zooplankton, such as jellyfish, are key players in these dynamics, yet long-term field records remain rare, limiting our ability to assess their ecological responses to climate change. Here, we analyzed a 13-year time series (2010-2023, excluding 2012) of Aurelia coerulea (von Lendenfeld, 1884) in Thau Lagoon, a French Mediterranean coastal ecosystem, based on bi-monthly sampling. Abundance and body size were quantified for both ephyra and medusa stages. Results revealed pronounced interannual variability: ephyrae and medusae were present for 33-199 and 0-112 days, respectively, with maximum densities reaching 1474 ind. 100 m^-3 in 2014 but only 14 ind. 100 m^-3 in 2018. The relationships between population dynamics, growth rates, phenology and environmental parameters (Sea Surface Temperature, salinity and chlorophyll a concentration) were investigated. Contrary to prevailing assumptions, our results showed that warming reduced bloom magnitude, accelerated bloom timing, and decreased growth rates. These findings reveal that not all jellyfish benefit from ocean warming, highlighting species- and population-specific thermal responses. By challenging the generalized narrative of climate-driven jellyfish proliferation, our study underscores the need for more nuanced perspectives on gelatinous plankton dynamics under climate change.